1. Field of the Invention
The present invention relates in general to temperature sensors, and in particular, to a new and useful optical pyrometer for measuring furnace gas temperatures.
2. Description of the Related Art
Prior devices have measured the temperature of entrained fly ash to determine gas temperature, limiting their application to only fuels with a high ash content. The present invention measures the temperature of actual constituent gases, thereby providing accurate temperature measurement with any fuel.
Radiation pyrometry, more commonly called optical pyrometry, measures the temperature of a substance by measuring the thermal radiation emitted by the substance.
Thermal radiation is a universal property of matter that is present at any temperature above absolute zero. Optical pyrometry utilizes the fact that the thermal radiation emitted by most substances is continuous over a spectral range of approximately 0.3 micron to 20 microns. This spectral range encompasses ultraviolet (UV) radiation, up to 0.38 micron; the visible (VIS) range, 0.38 to 0.78 micron; and infrared radiation (IR), 0.78 to 20.0 micron. IR radiation is further divided into three segments, near IR (0.78 to 3.0 micron), middle IR (3.0 to 6.0 microns), and far IR (above 6 microns).
The distribution of the thermal radiation of a substance over the spectral range is a function of both the temperature and emissivity of the substance. Higher temperatures shift the distribution toward the shorter wavelengths; lower temperatures shift the distribution toward the longer wavelengths. Higher emissivity increases the thermal radiation at any given temperature, whereas lower emissivity reduces the thermal radiation at the same temperature. A perfect thermal radiator is called a black-body, and has an emissivity of 1.0. Thermal radiators that are not perfect and that emit thermal radiation in the same spectral distribution as a black-body, but at reduced intensity, are called gray-bodies and have emissivities from zero to 1.0.
Emissivity can vary equally over the spectral range, called total emissivity; or it can vary as a function of the wavelength over the spectral range, called spectral emissivity. Both total emissivity and spectral emissivity can also vary as a function of the temperature of the substance. All of these emissivity variations can occur independently and simultaneously.
Scattering and absorption are two other important phenomena that affect intensity and distribution of the thermal radiation propagating through a medium, as well as the transmission distance (effective depth) through the medium. Scattering diffuses the thermal radiation, is highly dependent on both the species and physical size of the constituents of the medium, and increases singnificantly as the wavelength decreases. Absorption of thermal radiation is generally wavelength specific; the absorbed wavelength(s) being dependent upon the species that are exposed to the thermal radiation.
Optical pyrometry utilizes all of these radiating and propagating properties of matter to ascertain the temperature of a substance through a medium by measuring the intensity of the thermally radiated UV, VIS or IR energy of the substance. The radiating properties of matter are defined by Planck""s equation for spectral emissivity and the Stefan-Boltzmann law for total radiated energy. The propagating properties through a medium are defined by the Boujuar-Lambert law for absorption and the Rayleigh and Mie equations for scattering.
Optical pyrometers can be either narrow bandpass or broad bandpass instruments. Typically, narrow bandpass optical pyrometers utilize Planck""s equation to determine temperature, whereas broad bandpass optical pyrometers use the Stefan-Boltzmann law. Optical pyrometers are also classified according to the particular wavelength(s) utilized in determining the measured temperature, i.e., UV, VIS, or IR. Usually, the shorter wavelengths (UV, VIS and near IR) are used for higher temperature measurements, whereas longer wavelengths (middle and far IR) are used for lower temperature measurements.
Successful implementation of any optical pyrometer requires an extensive analysis of the application to determine the proper wavelength, bandpass and emissivity based on the characteristics of the measured substance over the desired measurement temperature range. Additionally, scattering and absorption in the propagating medium must also be considered to ensure that the measured thermal radiation is actually proportional to the measured temperature, and also to ensure the correct measurement depth into the propagating medium. This is especially important for boiler/furnace applications because the measured substance according to the present invention, flue-gas, is quite nebulous and its constituents can vary widely over normal operating conditions.
Without conducting the proper application specific analysis, most optical pyrometers cannot reliably and accurately measure boiler/furnace fireside gas temperatures. Typical VIS and broad bandpass IR type devices have a very short effective depth, and only measure the near-field temperature. Typical narrow bandpass IR type devices have an extremely long effective depth, and are greatly influenced by the opposite wall surface temperature.
U.S. Pat. Nos. 5,112,215 and 5,275,553, which are both incorporated here by reference, disclose optical pyrometers for measuring temperature based on single and double wavelength measurements of radiation from fly ash in a furnace.
The invention is an optical pyrometer which is based on Planck""s equation and which utilizes specific wavelengths to measure the actual temperature of constituent gases in the furnace and the convection pass section of a boiler. Specifically, the invention uses 1.38 micron infrared radiation to measure the temperature of H2O in a constituent gas that is a product of combustion of any hydrocarbon or carbon fuel. Alternative ranges of wavelengths are 1.3 micron 1.8 to 2.0 micron and 2.3 to 3.1 micron, providing temperature measurement of H2O, CO2, or mixtures of H2O and CO2.
The invention is primarily intended for applications on pulverized coal fired boilers, with boiler widths of 40 to 100 feet, and for all types and classes of coals. The standard calibration for the invention is made for measuring fireside gas temperatures from the furnace exit through the convection pass. With proper wall penetration hardware, the invention can be used on all normal forced draft, balanced draft, and induced draft boilers.
The invention can be used as a field diagnostic tool as well as a permanently installed on-line monitor. As a diagnostic tool, it can be used as a stand alone instrument, or along with data acquisition equipment to record data for extended tests. As a permanently installed on-line monitor, it can also stand alone or serve as an integral part of a larger system to implement automatic monitoring and control.
The invention, being an optical pyrometer, will measure temperature based on the light intensities that it sees. However, it is specifically tuned and calibrated to measure fireside gas temperature. Hence, it will not provide an accurate temperature reading of solid objects nor any other medium than fireside gas streams.
The invention measures the average temperature of a gas in a solid cone of about 6 degrees, along the line-of-sight of its lens-tube axis, over the entire length of the line-of-sight. The actual measurement contribution from any point within the cone of view varies with the fourth power of the temperature of the point; and also as a decreasing exponential into the gas stream depth. Hence, the average temperature indicated by the invention will be weighted toward the highest temperature in the field of view, and also slightly more toward the near-field rather than the far-field. The actual depth of measurement is also a function of optical scattering, consequently it will vary slightly with gas-stream particle load, gross unit, load, and excess air.
The invention requires an unobstructed field of view of the measurement area, especially in the near field. The 6 degree conical field-of-view equates approximately to a diameter of 5 feet at a distance of 50 feet. However, since the measurement contribution from any point decreases exponentially with distance, partial obstructions such as pendants and wing walls beyond a distance of 50 feet have very little effect on accuracy.
The accuracy of the invention is about +/xe2x88x9250 F. for normal pulverized coal fired utility boilers with furnace widths of 40 to 100 feet. Furnace widths of less than 40 feet may require special calibration to provide the specified accuracy. Furnace widths greater than 100 feet may require multiple units to provide full width temperature coverage.
Since the invention is tuned for specific gas species in the fireside gas stream, the opposite walls have very little influence on its measurement accuracy. Also, since the opposite walls are farthest from its objective lens, their effect is minimized by the exponential decrease of response with distance.
The usual measuring temperature range for the invention is 1000 F. to 2800 F. (approximate). Special calibration for other temperature ranges can be provided to lower the range to 700 F. minimum or increase the range to 3000 F. maximum, without degradation of measurement accuracy.
The invention is designed for installation at any location from the furnace exit, throughout the convection pass section. It can be installed at any standard furnace observation port, or any other wall penetration, i.e., sootblower or thermoprobe location, etc. A suitable wall box should be installed in the boiler wall for permanent installations.
For accurate measurement, the invention should be installed at a location with a 40 to 100 foot line-of-sight. For most boilers this is across the boiler width rather than from front wall to rear wall. Special care should be taken to avoid nearness to corners, pendants and wing walls that can restrict the 6 degree conical field-of-view in the near field. The optical head includes a lens for focusing infrared radiation from a line-of-sight cone of a solid angle of about 3 to 8 degrees from the furnace.
The invention is not intended to measure actual combustion temperature. Generally, the 1000 F. to 2800 F. measurement range will inherently prevent installation too close to the furnace combustion zone. However, for some applications, combustion temperatures can be less than 2800 F., and care should be exercised to ensure that the measurement zone is well clear of the combustion zone.
Since the average temperature indicated by the invention is weighted toward the highest temperature in the field of view, and also toward the near-field rather than the far-field, multiple units can be utilized to indicate temperature variation across the furnace width. For best results, the furnace width should be 50 feet or greater, and the units should be installed in pairs; one on each sidewall, on opposite ends of the same line-of-sight.
An object of the present invention is to provide a furnace pyrometer which is specially designed to achieve the foregoing functions in an effective manner.
A further object of the present invention is to provide a furnace pyrometer which is specially designed to sense the intensity of intermediate infrared radiation which is characteristic of one or more specific gas constituents generated by the combustion of hydrocarbon or carbon fuel including coal, natural gas, oil, organic matter or other combustible fuel.
A still further object of the present invention is to provide a pyrometer which is simple in design, rugged in construction, and economical to manufacture.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which the preferred embodiment of the invention is illustrated.